Does a CEL being active matter for mpg?

[dirtydave]

lol you looked a little too deep into that one hahaha

[Striker2237]

Okay so I was doing a huge amount of work to my truck today and noticed the morons who I had do my exhaust for me put the post cat sensor IN FRONT of the cat and the fuel air ratio sensor BEHIND the pre-cats.........mind you my truck should not even have pre-cats......so that means my computer is reading things slightly off (not much mind you as its well within normal values according to the DRBIII I have) and throwing the code because the sensor is on the wrong side of the cat....